1. Field of the Invention
The present invention relates to a non-aqueous secondary battery and a method for producing the same.
2. Description of Related Art
Much attention has been paid to the development of non-aqueous secondary batteries because they have a high voltage and a high capacity. Non-aqueous secondary batteries employ, as a negative electrode material (negative electrode active material), lithium (Li), a Li alloy, or a natural or artificial graphite carbon material capable of intercalating and deintercalating Li ions.
Recently, however, compact portable devices having multiple functions require higher capacity batteries. To meet such demand, materials capable of absorbing a larger amount of Li (hereinafter sometimes referred to as “high capacity negative electrode material”), such as low crystalline carbon, silicon (Si) and tin (Sn), are attracting a lot of attention as promising negative electrode materials.
One of the promising high capacity negative electrode materials for non-aqueous secondary batteries is a composite material (SiOx) having a structure in which Si ultrafine particles are dispersed in SiO2. In a battery containing this composite material, charge/discharge reaction proceeds smoothly because Si that reacts with Li is in the form of ultrafine particles. At the same time, because SiOx particles having the above structure have a small surface area, the composite material offers good spreadability when used as a coating material for forming a negative electrode material mixture layer. The composite material also offers good adhesion between a negative electrode material mixture layer and a current collector.
SiOx, however, is a low conductive material, and therefore when a negative electrode is formed using this material, SiOx should be thoroughly mixed/dispersed with a conductive auxiliary material. JP 2004-47404 A and JP 2005-259697A disclose a technique to cover the surface of SiOx with carbon to increase its conductivity before the formation of a negative electrode.
Because SiOx has a relatively large irreversible capacity, in order to achieve a higher capacity battery containing SiOx as a negative electrode material, it is preferable to introduce a Li source such as Li metal into the negative electrode material.
A method for introducing Li metal into a negative electrode material is to attach a Li metal foil onto the surface of a negative electrode material mixture layer containing the negative electrode material so as to introduce the Li metal contained in the Li metal foil into the negative electrode material contained in the negative electrode material mixture layer. However, the present inventors found the following through studies. When a Li metal foil is placed directly onto a negative electrode material mixture layer containing SiOx, because SiOx is highly reactive with Li, the Li contained in the Li metal foil enters the SiOx only upon contact of the Li metal foil with the negative electrode material mixture layer, causing an expansion of SiOx. Further, the introduction reaction of Li occurs locally and nonuniformly, so that the produced negative electrode is curved.
As a technique to introduce Li metal into a negative electrode material before the formation of a battery, JP 2000-182602A proposes to produce a battery using a negative electrode in which a support layer containing water-insoluble particles is formed between a layer containing a negative electrode material (negative electrode material mixture layer) and a Li metal foil. If the rate of introducing Li from the Li metal foil into the negative electrode material mixture layer is controlled by applying the technique disclosed in JP 2000-182602A and investigating the constitution of the support layer, even when SiOx is used as a negative electrode material, the negative electrode may not be curved.
However, the present inventors examined and found that even when a non-aqueous secondary battery is formed by applying the technique disclosed in JP 2000-182602A, when the positive electrode material comprises a Li-containing transition metal oxide, lithium dendrites are deposited on the negative electrode containing SiOx. As a result, the battery characteristics tend to be poor. This is presumably because the Li metal foil has a certain thickness (several tens of micrometers), a larger amount of Li in a metallic state is present in the battery than the amount required to be introduced into the SiOx, and the excess Li is deposited onto the negative electrode, forming dendrites. Because the Li metal foil is very soft, it is difficult to form it into a uniformly thin film or to previously control the amount of Li introduced into the negative electrode material.
According to the constitution disclosed in JP 2000-182602A, the negative electrode material comprises an Sn oxide having a large irreversible capacity, which requires a larger amount of Li than SiOx, and the positive electrode material (positive electrode active material) comprises a metal oxide originally containing no Li, which is capable of absorbing Li. It thus appears that the deposition of dendrites can be prevented even when a Li metal foil is used. However, in order to achieve a higher capacity non-aqueous secondary battery, it is preferable that a Li-containing transition metal oxide is used as a positive electrode material (positive electrode active material) rather than a metal oxide containing no Li, and therefore it is desired to achieve a much higher capacity battery by combining a positive electrode containing a Li-containing transition metal oxide with a negative electrode containing SiOx as a negative electrode material.